1. Field of the Invention
The invention relates to filters employed in the power circuitry of tape recording and/or playback systems. In the preferred embodiment, the invention relates to a ripple filter (i.e., choke-input or capacitor-input filter) adapted to filter the noise components created by the DC motor in a conventional multitrack, endless tape recording/playback system wherein the inductive component of said filter comprises the solenoid coil of the channel changing mechanism.
2. Description of the Prior Art
It is well known that the tape driving DC motor of a conventional tape recorder produces pulse-type noise components caused by rotor-to-brush arcing within the motor. The prior art tape recorders provide a number of circuit designs which are adapted to prevent these noise components from entering into the amplifier portion of the tape recorder circuitry thereby reducing the signal-to-noise ratio to acceptable levels for recording and high fidelity playback.
Heretofore, three types of noise reduction have been in practical use: (1) the use of a low noise level DC motor; (2) the use of an AC motor; and (3) the insertion of a filter network between the motor driving circuitry and the amplifier circuitry. The principal disadvantage of the first approach is that low noise level DC motors are costly. Furthermore, even with the best of these low noise level motors, the elimination of the noise is still incomplete and subject to increase upon wearing of the brush-rotor contact and not generally acceptable for high quality recording and playback. The use of an AC motor, which does not cause brush injection of current, effectively elminates the noise; however, this approach has the disadvantage of not being adaptable to battery operated recorders. An additional setback of the use of an AC motor is that it produces considerably more heat than an equivalent DC motor and, therefore, reduces the life of various electronic components within the recorder. The third approach of inserting the filter network between the DC motor and the amplifier circuitry accomplishes the noise elimination effectively in principle; however, in practice the optimum noise filtering requires an appropriate inductance-capacitance combination which in turn requires added parts and additional cost in manufacturing.
The three types of filter networks commonly used to filter out the noise produced by the DC motor are: (a) an inductance L-type filter in which an inductor is inserted in series between the DC power source and the tape driving motor and a capacitor is inserted parallel to the motor (see FIG. 4A); (b) a resistance L-type filter as in (a) above wherein the inductor is replaced by a resistor (FIG. 4B); (c) a pi-type filter in which an inductor is inserted in series between the DC power source and the tape driving motor and a capacitor is inserted on each side of the inductor (FIG. 4C); and (d) a pi-type filter having both a resistor and an inductor connected in series (FIG. 4D).
The efficiency of a filter network is proportional to the time constant of such a network. Hence, in principle, the greater the inductance and the capacitance, the more efficient the filter network. In practice, the cost of capacitors and inductors increases in proportion to the capacitance or inductance of such a component. In order to compromise between the filtering efficiency and the manufacturing cost, the inductor of a typical filter has been replaced with a relatively less expensive resistor and an inductor of less inductance. Unfortunately, the use of a resistor in the filter network results in a voltage drop across the resistor.
The use of a pi-type network has the disadvantage of requiring at least three components which results in an inescapable cost increase. It is believed that no prior art tape recorder provides an efficient DC motor noise filtering circuit without utilizing additional high inductance and high capacitance components in the inserted filter network. Thus, there is an acute need for a simple and efficient filtering circuitry which requires fewer additional costly components and achieves the high efficiency necessary for recording and high fidelity playback.
The typical eight-track, four-channel tape recorder has a solenoid coil and plunger arrangement connected in series with a channel changing switch. In such an arrangement, the switch supplies a momentary impulse to the solenoid coil which in turn activates its plunger. Through appropriate mechanical linkage, the plunger serves to move the magnetic head to the next channel and to index the channel indicator lights to indicate the appropriate channel. A more detailed description of conventional channel changing mechanisms can be found in numerous publications, including U.S. Pat. No. 3,564,157. The solenoid coils in present use are used only during the channel changing operation as a means for moving the plungers. Since such solenoid coils possess high inductance and are relatively expensive, it is highly desirable to incorporate these coils into the power circuitry of the recorder to perform other functions. It is believed that the prior art does not teach suitable means for using the solenoid coil for purposes other than channel changing.
It is a purpose of the present invention to incorporate the conventional channel changing solenoid coil into the power circuitry of a multitrack endless tape recorder as the inductive component in either a filter for the noise impulses from the DC motor or a filter for the DC wave rectifier.
Also well known in the art is that in recent years certain tape systems, in particular cassette systems, have been provided with an automatic reverse mechanism for reversing the direction of the tape at each end of the tape. Such reverse mechanisms are achieved typically by adhering a metal foil strip near the ends of the tape and providing a sensor to detect the strip, thereby actuating a solenoid coil-plunger arrangement for reversing the tape direction. It is a purpose of the present invention to utilize such a solenoid coil as the inductive component of a filter in the cassette system or other such tape system.
Another use of solenoid coil-plunger arrangements in tape recording systems is their use in ejecting tape cartridges. According to the present invention, it is also contemplated that such a solenoid coil may be used as the inductive component of a filter in the tape system.
The present invention recognizes that virtually any tape system having a solenoid coil and plunger arrangement useful as an electromechanical actuator during momentary current pulses through the solenoid can also utilize the solenoid as the inductance in a filter network.